Generally, a battery charging circuit operates in a constant current (CC) charging mode and a constant voltage (CV) mode. During a charging process, the battery charging circuit firstly charges a battery module in the CC charging mode until a battery voltage of the battery module reaches a certain level, then changes to CV charging mode to charge the battery module. Taking a lithium battery with a fully- charged voltage of 4.2 volts (V) as an example, the battery charging circuit first charges the lithium battery in a constant current charging mode. When the lithium battery is charged to 4.1V, the battery charging circuit will switch to the constant voltage charging mode to continue charging the battery voltage of the lithium battery to 4.2V. The above constant current charging mode is used to quickly charge the lithium battery. The constant voltage charging mode is used to avoid overcharging the lithium battery, thereby extending the life of the lithium battery.
FIG. 1 shows a schematic diagram of an existing battery charging circuit. As shown in FIG. 1, the existing battery charging circuit includes a charging transistor 12, a sensing transistor 14, and a current detector 16. The charging transistor 12 is coupled between a system terminal Vsys and a charging terminal Vbat. The system terminal Vsys has a system voltage generated by a power supply. The charging terminal Vbat has a battery voltage generated by the battery module BAT. Further, one end Al of the charging transistor 12 is coupled to the system terminal Vsys, and the other end A2of the charging transistor 12 is coupled to the charging terminal Vbat, and the control terminal A3 is controlled by the control signal Vg to generate a charging current Ich flowing through the charging transistor 12 according to the control signal Vg. The sensing transistor 14 is coupled between the system terminal Vsys and the current detector 16. Further, the sensing transistor 14 has one end B1coupled to the system terminal Vsys, the other end B2 coupled to the current detector 16, and the control end B3 controlled by the control signal Vg, to generate a sensing current Is flowing through the sensing transistor 14 according to the control signal Vg.
The current detector 16 is coupled between the sensing transistor 14, the charging terminal Vbat and the charging transistor 12, and configured to generate, according to the battery voltage, a sensing voltage Vs corresponding to the charging current Ich, and generate, according to the sensing voltage Vs and a reference voltage Vref, the control signal Vg, thereby controlling turned-on and turned-off states of the charging transistor 12 and the sensing transistor 14. Furthermore, the current detector 16 has a voltage level controller 16a, a sensing resistor 16b and an operational amplifier 16c. The voltage level controller 16a is coupled between the sensing transistor 14 and the sensing resistor 16b, so as to make the end A2 of the charging transistor 12 and the end B2 of the sensing transistor have the same voltage value (i.e., the charging voltage Vbat). The sensing resistor 16b converts the sensing current Is flowing through the sensing transistor 14 into a sensing voltage Vs. A positive input terminal of the operational amplifier 16c is coupled between the voltage level controller 16a and the sensing resistor 16b, and a negative input terminal of the operational amplifier 16c receives a reference voltage Vref. The output terminal of the operational amplifier 16c generates the control signal Vg according to the sensing voltage Vs and the reference voltage Vref, thereby controlling the turned-on and turned-off states of the charging transistor 12 and the sensing transistor 14.
As shown in FIG. 1, a control terminal A1 of the charging transistor 12 is coupled to a control terminal B1, and the end A2 of the charging transistor 12 has a voltage value (i.e., a voltage value of the charging voltage Vbat) that is the same as the voltage value of the end B2 of the sensing transistor 14. Therefore, the charging current Ich flowing through the charging transistor 12 has a proportional relationship with the sensing current Is flowing through the sensing transistor 14, and the proportional relationship mentioned above is determined according to a size ratio between the charging transistor 12 and the sensing transistor 14.
In the constant current charging mode under ideal conditions, the battery voltage gradually rises with time, and the charging current Ich is a constant current. However, non-ideal characteristics of the electronic components (e. g. transistors) will cause inaccurate turned-on voltage of the transistor (threshold voltage), so that the charging current Ich changes as time increases. For example, FIG. 2 is a diagram showing a relationship between the existing system voltage, battery voltage and charging current. The voltage of the system terminal Vsys is 3.65V and the battery voltage is increased from 3V to 3.58V. During 1.3 seconds (s) to 1.5 seconds, the charging current Ich may cause an error amount of, for example, 33% due to inaccuracy of the turned-on voltage. From the current ratio RAO between the charging current Ich and the sensing current Is (shown in FIG. 3), the current ratio RAO =charging current Ich/sensing current Is. Therefore, during 0 seconds to 1.3 seconds, the current ratio RAO is 21k or close to 21k, which represents that a ratio of the charging current Ich to the sensing current Is is a constant value. However, during 1.3 seconds to 1.5 seconds, the current ratio RAO is significantly reduced as the time increases, representing that the error ratio of the charging current Ich and the sensing current Is is increased.
Therefore, in the constant current charging mode, if the ratio of the charging current Ich to the sensing current Is can be maintained at a constant value or close to the constant value, the charging current Ich can be maintained at a constant current value as time increases, thereby accurately controlling the charging current Ich.